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June 2018 p33 q10
2143
The points A and B have position vectors \(2\mathbf{i} + \mathbf{j} + 3\mathbf{k}\) and \(4\mathbf{i} + \mathbf{j} + \mathbf{k}\) respectively. The line l has equation \(\mathbf{r} = 4\mathbf{i} + 6\mathbf{j} + \mu(\mathbf{i} + 2\mathbf{j} - 2\mathbf{k})\).
(i) Show that l does not intersect the line passing through A and B.
The point P, with parameter t, lies on l and is such that angle PAB is equal to 120°.
(ii) Show that \(3t^2 + 8t + 4 = 0\). Hence find the position vector of P.
Solution
(i) The vector equation of the line passing through A and B is \(\mathbf{r} = 2\mathbf{i} + \mathbf{j} + 3\mathbf{k} + \lambda(2\mathbf{i} - 2\mathbf{k})\).
Equating the components of the line l and the line through A and B gives:
\(4 + \mu = 2 + 2\lambda\)
\(6 + 2\mu = 1\)
\(-2\mu = 3 - 3\lambda\)
Solving these equations, we find that they are inconsistent, confirming that the lines do not intersect.
(ii) The direction vector for \(\overrightarrow{AP}\) is \((2 + t, 5 + 2t, -3 - 2t)\).
Using the cosine of the angle formula, \(\cos 120^{\circ} = -\frac{1}{2}\), we set up the equation:
Solving this quadratic equation, we find \(t = -2\) or \(t = -\frac{2}{3}\). Using \(t = -2\), the position vector of P is \(2\mathbf{i} + 2\mathbf{j} + 4\mathbf{k}\).